This invention relates to a coupling mechanism for coupling each of control rods inserted in the core of a nuclear reactor to a control rod driving apparatus.
The power of a nuclear reactor is controlled by the control rods inserted in and withdrawn from the reactor core. Insertion and withdrawing of the control rods in and from the reactor core are effected by the control rod driving apparatus which moves each control rod axially thereof. The control rod driving apparatus performs the function of controlling the rate of movement of each control rod, and is provided with means for inserting each control rod as quickly as possible in the reactor core when it is necessary to effect emergency shutdown of the reactor as the result of the occurrence of an accident in an installation or equipment associated with the reactor. More specifically, the control rod driving apparatus is provided with normal control rod inserting and withdrawing means for moving each control rod at a predetermined rate and emergency control rod inserting means for inserting each control rod at high speed to effect emergency shutdown of the reactor.
As is well known, prolonged use of a control rod results in a decline in the function thereof to control the power of a nuclear reactor. Thus old control rods are replaced by new ones usually at regular intervals. To this end, each control rod is coupled to the control rod driving apparatus by a coupling mechanism which allows the control rod to be detachably coupled to the control rod driving apparatus.
A primary requirement for the construction of a control rod coupling mechanism is that coupling and uncoupling of the control rod and the control rod driving apparatus can be effected readily without any trouble. This requirement should be met because, when the aforesaid control rod inserting means for inserting the control rod in the core in an emergency fails to operate successfully, it is necessary to uncouple the control rod from the control rod driving apparatus to permit the former to drop by gravity into the reactor core. Thus the control rod is arranged vertically. Moreover, the control rod is contaminated with radioactivity once it is inserted into the reactor core, so that manual operation of the coupling mechanism should be avoided.
One type of control rod coupling mechanism known in the art which meets the aforesaid requirement includes a hollow driving extension shaft extending from the control rod driving apparatus for axial reciprocatory movement and having a lower end portion divided peripherally into a plurality of pieces by forming therein a plurality of axially directed slits of a suitable length to provide a plurality of fingers each formed with an outwardly bulging portion changing smoothly in contour. The extension shaft has mounted in its hollow portion a cam which is adapted to move through the hollow portion for reciprocatory movement axially of the shaft to force the bulging portions of the fingers outwardly by a protruding portion of the cam. The fingers are restored to their original portions by the elastic force of restitution inherent in the fingers themselves when the cam is released from engagement with the fingers. To this end, the fingers are formed of a material of a small thickness so that they may have a suitable elastic restoring force. Meanwhile the control rod is formed at its upper end portion with an axial hole of a suitable length to allow the lower end portion of the extension shaft to be inserted therein, the hole being formed in its inner wall surface with grooves each adapted to receive therein one of the bulging portions of the fingers in engagement therewith. The driving extension shaft is arranged in axial alignment with the control rod, and is moved downwardly from this position to permit the forward end of the fingers to be received in the hole formed in the upper end of the control rod in a manner to allow the bulging portions of the fingers to be brought almost into engagement in the grooves. While the bulging portions of the fingers are in this position, the cam is actuated to force the bulging portions of the fingers outwardly by the protruding portion of the cam into intimate engagement in the grooves, thereby completing the coupling of the control rod to the control rod driving apparatus.
In the event that it is necessary to effect emergency shutdown of the reactor and the aforementioned means for quickly inserting the control rod in the reactor core in an emergency fails to function, the cam is actuated to release the force acting to force the bulging portions of the fingers outwardly so as to bring the bulging portions out of engagement in the grooves and permit the control rod to move downwardly by its own weight into the reactor core. U.S. Pat. No. 3,604,746 discloses a mechanism similar in construction to the aforementioned mechanism. The aforementioned type of coupling mechanism meets the requirement that the control rod should be readily coupled and uncoupled without trouble as stated previously.
The coupling mechanism for coupling the control rod to the control rod driving apparatus is inserted in the reactor core in which it is subjected to the irradiation of neutron rays. As a result, the materials of the mechanism are liable to deteriorate, and particularly the fingers which are formed to have a small thickness are markedly affected by the irradiation of neutron rays. Moreover, when the control rod is coupled to the driving apparatus, an axially directed force is exerted on the fingers as aforementioned, so that the fingers tend to undergo buckling. If this phenomenon occurs, not only the control rod but also the control rod driving mechanism itself or some of the component parts of the mechanism should be replaced, thereby interfering with the efficient operation of the nuclear reactor.
Thus when the coupling mechanism shows deterioration, there arises the necessity for replacing the old mechanism by a new one or using a new type of coupling mechanism which is free from the development of a buckling phenomenon due to deterioration of the material.